1. Field of the Invention
The present invention relates to an inverter in a lamp ignition device, particularly to a mono-stage high-efficiency inverter for the backlight module of a liquid crystal display (LCD).
2. Description of the Related Art
Discharge lamps, such as cold cathode fluorescent lamps (CCFLs), are usually used as the backlight source of LCD panels. Such a lamp has terminal voltage characteristics that vary with the immediate status and the frequency of the stimulus (AC signal) applied to the lamp. The CCFL will not conduct current until it is struck or ignited. When the lamp conducts current, the applied terminal voltage is less than the strike voltage. For example, the terminal voltage must be greater than or equal to 1500 V to strike the lamp. Once an electrical arc is strike in the CCFL, the terminal voltage falls to a lower run voltage, which is approximately one third of the strike voltage, and the current input range is relatively wide. For example, the run voltage of a CCFL may be 500 V with a current range of 500 mA to 6 mA while the strike voltage thereof is 1500 V. The CCFL is usually driven by an AC signal with a frequency ranging from 30 KHz to 100 KHz.
Discharge lamps exhibit negative resistance characteristics, so the operating voltage decreases when the consumed power increases. The circuit for supplying power to the lamp, such as an inverter, requires a controllable alternating current power supply and a feedback loop capable of accurately monitoring the current in the lamp so as to maintain the stability of the circuit and to have load-regulation ability.
When designing inverters for LCD backlight system of notebook or desktop computers, efficiency, cost and size are some of the most critical factors. A conventional inverter for LCD backlight system, such as the inverter numbered CXA-K05L-FS sold by TDK Corporation of Tokyo, Japan, comprise a buck converter and a current-fed self-oscillating push-pull inverter (also called a Royer inverter). The efficiency of such combination of a buck converter and a Royer inverter is limited by the two power conversion stages. Particularly, the magnetizing inductance of the transformer in the Royer DC/AC converter serving as the resonant inductance causes additional power loss.
Currently, the efficiency of an inverter having a structure of two power conversion stages, the buck stage and the Royer stage, is about 70-80%. Especially in the case of a low input voltage, a higher coil ratio of the transformer is required, so that the loss increases and the entire efficiency decreases. Such transformer structure uses a central tap, and thus is difficult to be miniaturized and has a higher manufacturing cost. Besides, only one set of the coils operates in each of the half-cycle of the transformer, and the utility rate is accordingly low. Moreover, the output voltage waves of such inverter have higher harmonic compositions, which cause a lower illuminating efficiency, a shortened lifespan of a lamp, and a greater electromagnetic interference. In summary, such inverter has the disadvantages of higher manufacturing cost, lower efficiency, and excessive harmonic waves.
In view of the above, it is an object of the present invention to provide an inverter for lamp ignition device, which is constructed by a single power conversion stage.
It is another object of the present invention to provide an inverter for lamp ignition device, in which the transformer has a simple structure without the provision of a central tap.
It is a further object of the present invention to provide an inverter for lamp ignition device, which operates at a duty cycle of approximately D=0.5 and is dimmable by burst mode control. Current asymmetry is thus avoided.
It is still a further object of the present invention to provide a lamp ignition system, which outputs to the lamp voltage waves with less harmonic compositions. Therefore, higher illuminating efficiency, longer lifespan of the lamp, and smaller electromagnetic interference are achieved.
Accordingly, the invention discloses an inverter for the ignition of a discharge lamp. The inverter according to the invention comprises a transformer, a first switch transistor, a second switch transistor, a reset capacitor and a control circuit. One of the source/drain of the first switch transistor is electrically coupled to the primary side of the transformer. One of the source/drain of the second switch transistor is electrically coupled to the primary side of the transformer. The reset capacitor is electrically coupled between the other of the source/drain of the first switch transistor and the other of the source/drain of the second switch transistor. The control circuit controls the first switch transistor and the second switch transistor not to conduct current at the same time.
The control circuit further renders both the first and the second switch transistors non-conducting during the interval between the conducting of the first switch transistor and the conducting of the second switch transistor. The control circuit further controls the current value at the secondary side of the transformer according to a burst mode control signal.
The control circuit comprises a driving circuit which utilizes the voltage across the reset capacitor as driving power for generating two switch control signals respectively output to the first switch transistor and the second switch transistor so as to reduce the conducting resistance thereof.
Moreover, the present invention discloses a lamp ignition system comprising a discharge lamp and an inverter. The inverter comprises a transformer, a first switch transistor, a second switch transistor, a reset capacitor, a first snubber capacitor, a second snubber capacitor and a control circuit. The secondary side of the transformer is electrically coupled to the discharge lamp. One of the source/drain of the first switch transistor is electrically coupled to the primary side of the transformer. One of the source/drain of the second switch transistor is electrically coupled to the primary side of the transformer. The reset capacitor is electrically coupled between the other of the source/drain of the first switch transistor and the other of the source/drain of the second switch transistor. The first snubber capacitor is electrically coupled between the source and the drain of the first switch transistor. The second snubber capacitor is electrically coupled between the source and the drain of the second switch transistor. The control circuit generates two switch control signals in response to a voltage feedback signal representing the current value at the secondary side of said transformer and respectively outputs them to the gate of the first switch transistor and the gate of the second switch transistor to thereby cause the first switch transistor and the second switch transistor not to conduct current at the same time.
The control circuit comprises an error amplifier and a pair of comparators. The error amplifier senses the voltage feedback signal representing the current value of the discharge lamp and a reference voltage to perform error amplification. The pair of comparators generate two switch control signals according to the comparison result of the output of the error amplifier and a reference triangular wave.